1. Field of the Invention
The present invention relates to a vacuum arc vapor deposition apparatus and a vacuum arc vapor deposition method used for forming a thin film of excellent lubricating property and hardness on such a substrate as an automobile part, machine part, machine tool, and metal mold, which includes a magnetic coil for guiding a plasma produced by a vacuum arc evaporating source to the vicinity of the substrate. More particularly, the invention relates to a vacuum arc vapor deposition apparatus for preventing degradation of the uniformity of a thickness distribution on a surface of the substrate, which is caused by the drift of the plasma in a magnetic field developed by the magnetic coil.
2. Description of the Related Art
A vacuum arc vapor deposition apparatus forms a film (thin film) on a substrate by using a vacuum arc evaporating source which vaporizes a cathode by vacuum arc discharge to produce a plasma containing a cathode material. The vacuum arc vapor deposition apparatus is advantageous in that a film forming rate is high and highly productive.
The cathode material vaporized from the cathode of the vacuum arc evaporating source contains macro particles (called droplets) of several μm or larger in addition to micro particles suitable for film formation. The macro particles fly to and attach onto the surface of the substrate, possibly damaging the adhesion property and smoothness (surface roughness) of the film.
To solve the above problems, the following two techniques are already proposed: 1) technique for transporting the plasma to the substrate after the macro particles are removed from the plasma by the utilization of a deflection magnetic field (e.g., Japanese Patent Unexamined Publication No. 2001-3160), and 2) technique to make the macro particles fine by converging the plasma by the magnetic field to increase density of the plasma (e.g., Japanese Patent Unexamined Publication No. 2000-34561).
FIG. 10 is a cross sectional view showing a vacuum arc vapor deposition apparatus which uses the technique 1) above. The vacuum arc vapor deposition apparatus includes a film forming chamber (or vacuum chamber) 2 which is vacuum discharged by a vacuum discharging apparatus (not shown). A holder 8 for holding a substrate 6 on which a film is formed is located in the film forming chamber.
In this example, a gas 4, such as inactive gas or reaction gas, is introduced from a gas source (not shown) into the film forming chamber 2.
A bias voltage VB of −50 V to −500 V, for example, is applied from a bias power source 10 to the holder 8 and the substrate 6.
The film forming chamber 2 is connected to a vacuum arc evaporating source 12 through a pipe 28 (deflection pipe) bent about 90° in this example.
The vacuum arc evaporating source 12 includes a cathode 14 mounted on an end plate 29 of the pipe 28 with an insulating material 20 inserted therebetween. The cathode 14 is vaporized through vacuum arc discharge occurring between the cathode 14 and the pipe 28 serving also as an anode to produce a plasma 18 containing a cathode material 16. An anode electrode may be individually provided. Here, the “cathode material” means material forming the cathode 14. An arc discharging voltage is applied from an arc power source 22 to between the cathode 14 and the pipe 28. The vacuum arc evaporating source 12 includes a known trigger mechanism, a water cooling mechanism and the like. Those mechanisms are not illustrated in the specification, for simplicity.
A plurality of magnetic coils 24 are provided around an outer periphery of the pipe 28. The magnetic coils generate a magnetic field for deflecting the plasma 18 produced by the vacuum arc evaporating source 12, and guides (transports) the plasma 18 to the vicinity of the substrate 6 in the film forming chamber 2. Some of magnetic field lines 26 generated by the magnetic coils 24 are roughly illustrated in the figure, and as shown, those magnetic field lines extend substantially along an inner surface of the pipe 28. Those magnetic coils 24 are connected in series, and fed with a coil current IC for generating the magnetic field from a coil power source 30.
The plasma 18 produced by the vacuum arc evaporating source 12 is bent to substantially along the magnetic field lines 26 and transported to the substrate 6. The macro particles emitted from the cathode 14 are electrically neutral or negatively charged in the plasma 18. A mass of the macro particle is considerably large. Accordingly, those particles go straight irrespective of the magnetic field, and hit the inner wall of the bent pipe 28 and hence fail to reach the substrate 6. As a result, the plasma 18 little containing the macro particles is led to the vicinity of the substrate 6. Thus, it is prevented that the macro particles attach to the substrate 6. The apparatus which has the magnetic coils 24, pipe 28 and coil power source 30 (coil power source 40 in FIG. 1) as mentioned above is also called a magnetic filter where attention is put on the macro-particle removing function.
Ions (i.e., ionized cathode material 16) in the plasma 18 thus transported to near the substrate 6 are attracted to the substrate 6 under the bias voltage VB and the like, and deposited on the surface of the substrate to form a thin film on the substrate. When a reaction gas which reacts with the cathode material 16 to form a chemical compound is used for the gas 4, a compound thin film may be formed.
When an electron is transported in a uniform magnetic field, as well known, the electron makes a gyrating movement such that it winds round the magnetic field lines, under Lorentz forces given by the following equation 1. In the equation, q is a charge, v is an electron velocity, and B is a flux density (The same rule applies correspondingly to the description to follow.).F=qvB  [Equation 1]
Accordingly, in a uniform magnetic field, electrons emitted from two positions P and Q shown in FIG. 11 move along magnetic field lines 26 uniformly distributed, reach the substrate 6, and are incident on positions near positions P1 and Q1 corresponding to the positions P and Q.
Actually, a magnetic field developed by the magnetic coils 24 is not uniform and has gradients of a magnetic field without exception. For drift of charged particles, such as electrons, in a magnetic field having gradients, reference is made to “Newest Plasma Production Technique”, by Yoshinobu Kawai, published by IPC corporation on 5 Aug., 1991, pages 12 to 21. As described, the charged particle drifts at a drift velocity VD given by the following equation 2. In the equation, μ is magnetic permeability, ∇B is a gradient (vector) of the magnetic field, and Bv is a magnetic field (vector), and other things are the same as mentioned above. ∇ is a nabla or Hamiltonian operator.VD=−μ(∇B×Bv)/(qB2)  [Equation 2]
The gradient of the magnetic field will be discussed by using an apparatus which transports the plasma 18 by use of the deflection magnetic field as shown in FIG. 10 (or FIG. 1 to be described later).
A case where the magnetic coil 24 and the pipe 28 are circular in cross section is shown in FIGS. 12 to 18. In FIGS. 12 to 15, the cathodes 14a and 14b are simply represented by two positions “P” and “Q” (the same thing is correspondingly applied to the illustrations of FIGS. 19 to 21 to be described later). In FIGS. 16 to 18, the cathodes 14a and 14b are specifically illustrated (the same thing is correspondingly applied to the illustrations of FIGS. 22 and 23 to be described later and FIGS. 2 to 7).
In this case, the nature of the circular magnetic coils 24 gives the magnetic field in the pipe 28 such a gradient ∇B as shown in FIG. 14 that, an intensity of the magnetic field is lowest at the center 28a of the pipe inside, and gradually increases toward the outside. In a case where a plurality of magnetic coils 24 are disposed while being bent as shown in FIG. 10, for example, the lowest intensity of the magnetic field is located at a position somewhat outwardly shifted from the center 28a, actually.
Accordingly, as shown in FIGS. 12 and 13, electrons 32a and 32b emitted from the two positions P and Q drift at a drift velocity VD in the circumferential direction (FIG. 15) by the gradient ∇B of the magnetic field (FIG. 14), as defined in the equation 2. Therefore, the electrons land on the substrate 6 at positions shifted in the circumferential direction. The same thing is true for the ions, and hence the plasma drifts, while being shifted in the circumferential direction.
In a case of FIGS. 16 and 17 where two vacuum arc evaporating sources 12 are vertically spaced from each other and arranged along the z-axis, plasma 18 produced by the cathodes 14a and 14b reaches the substrate 6 while drifting in the circumferential direction. A density distribution of each the plasma produced by the cathodes 14a and 14b is usually depicted in a shape of an outward curve; the density is highest at the center of the plasma in cross section and gradually decreases toward its fringe. Accordingly, peaks 36a and 36b and fringes 38a and 38b of a film thickness distribution (viz., a film forming velocity distribution) appear on the surface of the substrate 6 as shown in a FIG. 18 instance. As shown, those peaks and fringes are located at positions shifted in the circumferential direction from positions 34a and 34b corresponding to the cathodes 14a and 14b. 
A case where the magnetic coils 24 and the pipe 28 are rectangular in their cross section is illustrated in FIGS. 19 to 23.
A magnetic field within the pipe 28 has such a gradient ∇B as shown in FIG. 20 that an intensity of the magnetic field is lowest at a part 28b slightly closer to the outside than the center 28a and gradually increases toward the outside. The gradient ∇B depends on the nature of the rectangular magnetic coils 24 and the arrangement of the plurality of magnetic coils 24 arranged while being bent as shown in FIG. 10 and the like.
As shown in FIGS. 12 and 19, electrons 32a and 32b emitted from two positions P and Q drift at a drift velocity VD, as defined in the equation 2 (FIG. 21), in a downward and oblique direction, which is the resultant of the downward direction and the lateral direction, by the gradient ∇B of the magnetic field 8 (FIG. 20).
In a case of FIGS. 16 and 22 where two vacuum arc evaporating sources 12 are vertically spaced from each other and arranged along the z-axis, plasma 18 produced by the cathodes 14a and 14b reaches the substrate 6 while drifting in the downward and oblique direction. Accordingly, peaks 36a and 36b and fringes 38a and 38b of a film thickness distribution appear on the surface of the substrate 6 as shown in a FIG. 23 instance. As shown, the peaks 36a and 36b and fringes 38a and 38b are located at positions shifted in the downward and oblique direction from positions 34a and 34b corresponding to the cathodes 14a and 14b. 
Actually, a shift of the peak 36a is different from that of the peak 36b. The lateral and downward shifts of the peak 36b on the lower side (as viewed in the z-axis, the same will apply hereinafter.) are greater than that of the peak 36a on the upper side. This fact was empirically confirmed. The peaks 36a and 36b are shifted in directions in which the distance between them increases. Such an example is illustrated in FIG. 23. Where such shifts occur, film formation little occurs at the central part of the substrate 6. Further, the shifts become larger as a distance of the substrate 6 from the vacuum arc evaporating source 12 increases.
Where the peaks 36a and 36b and the fringes 38a and 38b of the film thickness distribution on the surface of the substrate 6 are shifted by the gradient ∇B of the magnetic field, it is difficult to form a film on the substrate 6 as desired. The shift will deteriorate the uniformity of the thickness distribution on the surface of the substrate 6. When comparing with a case where the magnetic coils 24 and the pipe 28 are circular in cross section, in a case where the where the magnetic coils 24 and the pipe 28 are rectangular in cross section, the peaks 36a and 36b of the thickness distribution are shifted apart away from each other, and the shifts of them become larger as a distance between the substrate 6 ad the vacuum arc evaporating source 12 increases. Accordingly, the uniformity of the thickness distribution on the surface of the substrate 6 is more deteriorated.